1. Field of the Invention
The present invention relates to a dielectric filter, a dielectric duplexer, and a communication device for use in the microwave or millimeter wave range.
2. Description of the Related Art
In recent years, with the increasing popularity of mobile communications systems and multimedia, there are increasing needs for high-speed and high-capacity communications systems. As the quantity of information transmitted via these communications systems increases, the frequency range used in communications is being expanded and increased from the microwave range to the millimeter wave range. Although TE.sub.01 .delta.-mode dielectric resonators, which are widely used in the microwave range, can also be used in the millimeter waver range, extremely high accuracy is required in production because the resonance frequency of TE.sub.01 .delta.-mode dielectric resonators is determined by the outside dimensions of the cylindrical dielectric. However, because of contraction which occurs during the process of firing a dielectric material, it is impossible to produce a cylindrical dielectric having dimensions exactly corresponding to a desired resonance frequency. In the case where a dielectric filter is produced by disposing a plurality of TE.sub.01 .delta.-mode dielectric resonators in a metal case so that they are spaced a particular distance apart from each other, a high positioning accuracy is required because the degree of coupling between a dielectric resonator and input/output means such as a metal loop or between dielectric resonators is determined by the distance between these elements.
To solve the above problems, the inventors of the present invention have proposed, in Japanese Unexamined Patent Publication No. 8-265015, a dielectric resonator with a high dimensional accuracy and also a dielectric filter with a high positioning accuracy.
FIGS. 8 and 9 illustrate the basic structure of the dielectric resonator disclosed in the patent application cited above. FIG. 8 is an exploded perspective view of the dielectric filter according to this patent application, and FIG. 9 is a cross-sectional view taken along line X--X of FIG. 8.
As shown in FIGS. 8 and 9, the dielectric filter 110 includes a dielectric substrate 120, an upper conductive case 111, and a lower conductive case 112.
The dielectric substrate 120 is made up of a substrate having a particular relative dielectric constant. One principal surface of the dielectric substrate 120 is entirely covered with an electrode 121a except for two circular-shaped openings 122a having a particular size formed in the electrode 121a, and the other principal surface is entirely covered with an electrode 121b except for two circular-shaped openings 122b having a particular size formed in the electrode 121b. The openings 122a and 122b are formed at corresponding locations on the opposite principal surfaces.
The upper conductive case 111 is formed of metal in a box shape whose lower side is open. The upper conductive case 111 is disposed near the openings 122a of the electrode 121a in such a manner that the upper conductive case 111 is spaced by the dielectric substrate 120.
The lower conductive case 112 is made up of a metal plate bent at right angles at both sides. Dielectric strips 113a and 113b are disposed on both ends of the lower conductive case 112.
The dielectric strips 113a and 113b are located between the upper conductive case 111 and the lower conductive case 112 so that they act as NRD (non-radiative dielectric) transmission lines. Furthermore, as shown in FIG. 8, the dielectric substrate 120 is disposed on the dielectric strips 113a and 113b in such a manner that the ends of the respective dielectric strips 113a and 113b overlap the corresponding openings 122b on the other principal surface of the dielectric substrate 120. The dielectric strips 113a and 113b also serve as spacers by which the dielectric substrate 120 is spaced a fixed distance apart from the inner surface of the bottom of the lower conductive case 112.
In this structure, electromagnetic energy is confined substantially to the portions of the dielectric substrate 120 between the two opposite openings 122a and 122b formed in the electrodes 121a and 121b, respectively, and thus these two portions of the dielectric substrate 120 act as resonators. As a result, a dielectric filter having two stages of resonators is obtained.
In the structure described above, the resonance regions are defined by the sizes of the openings formed in the electrodes. Because openings having extremely high dimensional accuracy may be formed for example by means of etching, it is possible to realize a dielectric filter with resonators which are formed with high dimensional accuracy with respect to the resonance frequency and which are positioned with extremely high accuracy relative to each other. Furthermore, in the resonators of the dielectric filter 110, electromagnetic energy is very tightly confined substantially to the portions of the dielectric substrate 120 between the two openings 122a and 122b, and thus the resonators have high unloaded Q.
However, in the dielectric filter 110, the extremely tight confinement of electromagnetic energy results in weak coupling between adjacent resonators, and the weak coupling between adjacent resonators results in a narrow bandwidth.
More particularly, when the dielectric substrate 120 was made up of a single-crystal sapphire substrate with a thickness of 0.33 mm and a relative dielectric constant of 9.3, the openings 122a and 122b were formed so that they have a diameter of 3.26 mm and so that the distance between the adjacent openings 122a and the distance between the adjacent openings 122b are both 0.4 mm, the distance between the ceiling of the upper conductive case 111 and the inner surface of the bottom of the lower conductive case 112 was set to 3.2 mm, the resultant dielectric filter 110 with a center frequency of 60 GHz had a coupling coefficient lower than 0.5% and the rejection band width was as narrow as about 120 MHz.
It is possible to expand the bandwidth of such a filter by decreasing the distance between resonators (the distance between the adjacent openings 122a and the distance between the adjacent openings 122b) thereby increasing the coupling coefficient. However, in practice, there is a lower limit on the distance between resonators, and more specifically, the practical lower limit is about 0.1 mm. Even when the distance between resonators was reduced to the practical lower limit, the coupling coefficient was still as low as 1.5% and the bandwidth was as narrow as 360 MHz.
When the reduction in the distance between resonators is achieved by reducing the distance between the adjacent openings 122a or the distance between the adjacent openings 122b, it is required to perform a difficult patterning process on the electrode 121a or 121b.
Another problem is weak external coupling between the resonators and the input/output NRD dielectric strips 113a and 113b. To achieve required external coupling, it is required to optimize the positions of the two openings 122b formed in the electrodes on the other principal surface of the dielectric substrate 120 relative to the positions of the dielectric strips 113a and 113b. However, such optimization is difficult.
In view of the above, it is an object of the present invention to provide a resonator that can be easily coupled to an adjacent resonator or to input/output means. It is another object of the present invention to provide a filter having a wide bandwidth.